As an apparatus for supplying electric power, a DC-DC converter is a power supply apparatus which generally widely is used. The DC-DC converter can not only change a specific input voltage to a desired voltage but also supply a constant output voltage without being influenced by a change in input voltage, and supply a constant voltage regardless of a magnitude of a load of an output.
One of the most important factors representing performance of a DC-DC converter is conversion efficiency. Here, the conversion efficiency is a ratio of an electric power finally transferred to an output to an input electric power. In this case, as electric power is lost or used in various forms during a process of outputting an electric power obtained from an input, it is a better way to reduce the lost or used electric power in order to increase conversion efficiency.
Meanwhile, one of the most important factors representing a structure of a DC-DC converter is a topology. Topologies are largely classified into a buck topology, a boost topology, and a buck-boost topology. The buck topology is also called a step down, and is used when an output voltage is low as compared with an input voltage. Meanwhile, the boost topology is also called a step up, and is used when an output voltage is high as compared with an input voltage. However, since a relationship between the magnitudes of an input voltage and an output voltage is reversed in the buck-boost topology, a boost topology cannot be used in an input/output voltage condition where a buck topology is used, whereas a buck topology cannot be used in an input/output voltage condition where a boost topology is used.
In order to solve this problem, there exists a buck-boost topology called a step up-down. The buck-boost topology has structural advantages of the buck topology and the boost topology, and may be used regardless of a relationship of the magnitudes between the input/output voltages. There are various methods to realize the buck-boost topology. There is most basically a method of directly connecting the buck topology and the boost topology. However, in the above-mentioned structure, conversion efficiency lowers as electric power loss is increased by a transistor.
According to the related art, a DC-DC converter IC may be selected to be used according to magnitudes of input/output voltages, but the DC-DC converter needs to be changed if a relationship in the magnitudes between the input/output voltages is changed, or a board including the DC-DC converter IC even needs to be changed. Thus, a DC-DC converter including one DC-DC converter IC operated regardless of a relationship between magnitudes of input/output voltages while maintaining high efficiency is required.